The present invention relates to a thin-film of manganese oxide having a perovskite type crystal structure exhibiting metal-insulator phase transition and a process for producing the same thin film and also relates to a thermal type far-infrared sensor (bolometer) having high sensitivity and a wide band of effective wavelengths, which is produced by using the same thin-film material.
Materials having a large change in resistivity, which are well-known as metal-insulator phase transition (Physics of Metal and Non-metal (2nd ed.), written by Mott, translated by Ono and Otsuki and published by Maruzen in Japan, 1998) have been studied for possible various applications. Typical examples of those materials are high-temperature superconductors or transition metal oxides such as V2O3 and so on. The former has been developed for past ten several years to possess high quality and an improved transition range of 0.1K (90% of the temperature width from a maximal value to a least value in resistivity). However, a highest transition temperature is 136K for HgBa2Ca2Cu3Oy, which is far from a room temperature. The latter include V2O3 doped with Cr by 1%, which has a transition temperature close to a room temperature and exhibits a change of 2 orders of magnitude in resistivity. However, the transition is first-order and has a large hysteresis.
A thermal far-infrared sensing element is one of the devices using material having a big change in resistivity at a phase transition point. Conventional infrared sensing elements that have been developed are classified mainly into quantum type elements and thermal type elements. The quantum type elements use inner quantum effects such as photoconductive effect, photoelectric effect and photomagnetic effect of semiconductors, while the thermal type elements are used for detection of a temperature change occurred by conversion infrared rays into heat energy. The infrared sensing elements are widely used for transmission of signals between electronic devices, detection of a human body, monitoring of food cooking temperatures, medical diagnostics, security and crime prevention systems, thermal management, volcano monitoring, earth environmental monitoring, resource investigation, space observation and so on. Recently, many studies have been actively conducted for application of a wavelength band from infrared to middle millimeter waves (ultra-far-infrared rays) for communications.
However, the above quantum type infrared sensing elements have a narrow usable wavelength band since a detectable infrared wavelength band is limited by a used semiconductor. For detecting a long far-infrared of about 10 microns in wavelength, it is necessary to reduce bandgap energy or impurity ionization potential of the semiconductor used. Therefore, the sensing elements are required to be cooled down to the liquid helium temperature (4.2K) to prevent the increase of carriers that may be produced in a semiconductor from thermal excitation. The thermal type sensing elements have wide effective wavelengths but low detection sensitivity.
Recently, there have been studied infrared sensing elements using insulator-metal phase transition (or semiconductormetal phase transition) of compound perovskite type structure manganese oxide (for example, p. 593, Extended Abstracts (The Autumn Meeting, 1997); The Japan Society of Applied Physics). The infrared sensing element of this type has a large figure of merit (|d log xcfx81/dT |), where xcfx81 is electrical resistivity and T is absolute temperature, since the phase transition from insulator (or semiconductor) to metal can occur abruptly for a temperature change. Furthermore, it has a wide effective wavelength band relative to incident infrared radiation.
However, a normal compound perovskite type manganese oxide can maintain a large figure of merit merely within a low temperature range and has a decreased figure of merit at temperatures achieved by thermoelectric cooling means or at a room temperature. For example, a representative perovskite type manganese oxide La0.7Ca0.3MnO3 has a merit figure of about 0.16Kxe2x88x921 at a temperature of 255K. A merit figure of manganese oxide La0.7Ba0.3MnO3 having a transition point of 340K is decreased to 0.04 and a merit figure of manganese oxide La0.7Sr0.3MnO3 having a transition 380K is decreased to 0.02.
An object of the present invention is to provide a thin film of manganese oxide, which has a metal-insulator phase transition obtainable at a higher temperature than i-conventional material and is based upon the inventor""s studies on doping of the above material with carrier supplying alkali earth metals (such as Sr and Ca) and oxygen.
Another object of the present invention is to provide a process of producing the above manganese oxide thin-film.
Still another object of the present invention is to provide an infrared sensor made of the above manganese oxide thinfilm, which has the increased detection sensitivity and does not require cooling or requires a small cooling load. Means for solving problems
For accomplishing the above objects, inventors of the present invention have advanced studies on doping with alkali earth metals (Sr, Ca and so on) and oxygen, which are elements composing a perovskite type manganese oxide and supply with carriers and found thin film of perovskite type structure manganese oxide whose metal-insulator transition point obtainable within a very high temperature range.
The inventors have also found the fact that perovskite type manganese thin film prepared by a sol-gel process is given a higher phase transition temperature in comparison with a thin film prepared by an other method or has a metal-insulator transition range extended into a light dope side.
A thin film of perovskite type manganese oxide containing an element Ca or Sr and elements La, Mn and O, which is a first aspect of the present invention, is characterized in that a metal-insulation phase transition point of the thin film exists within a range of temperatures obtainable by a thermoelectric cooling method.
The range of thermoelectrically obtainable temperatures is a range of temperatures of xe2x88x9260xc2x0 C. (low temperature) to room temperature that can be achieved by thermoelectric cooling method using a thermoelectric cooler such as a Peltier element.
A thin film of perovskite type manganese oxide containing an element Ca or Sr and elements La, Mn and O, which is a second aspect of the present invention, is characterized in that an oxygen component ratio is homogeneous in the film owing to inhibition of diffusion of oxygen elements.
A thin film of perovskite type manganese oxide whose composition is expressed as La1xe2x88x92xCaxMnO3+xcex4 and which is a third aspect of the present invention, is characterized in that the Ca-component ratio x lies in a range of 0.1-0.2 and metal-to-insulator phase transition can occur within a temperature range obtained by thermoelectrically cooling method. The film thus constructed can work effectively at a room temperature or with a Peltier cooler having a small cooling load. The use of this film can create a high sensitivity infrared sensor having a high figure of merit.
A thin film of perovskite type manganese oxide whose composition is expressed as La1xe2x88x92xCaxMnO3+xcex4 and which is a fourth aspect of the present invention, is characterized in that the film has Ca component ratio x of 0.1 to 0.5 and the oxygen-component ratio (3+xcex4) is controlled to cause metal-to-insulator phase transition to occur in a range of thermoelectrically obtainable temperatures.
A thin film of perovskite type manganese oxide whose composition is expressed as La1xe2x88x92ySryMnO3+xcex4 and which is a fifth aspect of the present invention, is characterized in that Sr-component ratio y exists within a range of 0.05-0.185 and the metal-to-insulator transition can occur therein at temperatures obtainable by a thermoelectric cooling method.
A thin film of perovskite type manganese oxide whose composition being expressed as La1xe2x88x92ySryMnO3+xcex4 and which is a sixth aspect of the present invention, is characterized in that the film having Sr-component ratio within a range of 0.185 to 0.25 is free from the occurrence therein structure phase transition.
A thin film of perovskite type manganese oxide, which is a seventh aspect of the present invention, is characterized in that the thin film is prepared by a sol-gel method. The thin film prepared by the above method can effectively work at a room temperature or by cooling with a Peltier element having a small cooling load. The use of this film can produce an infrared sensor having high sensitivity at a low cost.
A thin film of perovskite type manganese oxide, which is an eighth aspect of the present invention, is characterized in that the thin film is prepared through a rapid thermal annealing step in a production process. The production process can be considerably shortened in time to effectively produce the same films at a reduced cost.
A process for production of a thin film of perovskite type manganese oxide containing an element Ca or Sr and elements La, Mn and O, which is a ninth aspect of the present invention and which film is of any of the first and second aspect of the present invention, is characterized in that a rapid thermal annealing process is conducted in an atmosphere containing at least excess oxygen. A further aspect of the present invention is a process for producing the above perovskite type manganese oxide thin-film, which is characterized by repeating the above rapid thermal annealing process plural times. Still further aspect of the present invention is a process for producing the perovskite type manganese oxide thin-film, which is characterized in that the rapid thermal annealing process is repeated plural times but a temperature of the second time process and thereafter is lower than that of the first time process. Another aspect of the present invention is a process for producing a perovskite type manganese oxide thin-film by using the rapid thermal annealing process, which is characterized by conducting the rapid thermal annealing process with a cooling rate of not more than 3xc2x0 C./min.
Finally, it is possible to provide a high-sensitivity thermal (bolometer) type infrared sensor comprising a readout circuit and a sensing portion, wherein the sensing portion has as its bolometer film the thin film of perovskite type manganese oxide.